CVE-2026-0877: Firefox DOM Security Bypass Vulnerability

CVE-2026-0877 Overview

CVE-2026-0877 is a mitigation bypass vulnerability in Mozilla Firefox's DOM: Security component. This security flaw allows attackers to circumvent built-in browser security controls designed to protect users from malicious web content. The vulnerability requires user interaction (such as visiting a crafted webpage) but can be exploited remotely over the network without authentication.

Critical Impact
Successful exploitation could allow attackers to bypass DOM security mitigations, potentially leading to high-impact confidentiality and integrity breaches without requiring any privileges.

Affected Products

Firefox < 147
Firefox ESR < 115.32
Firefox ESR < 140.7

Discovery Timeline

January 13, 2026 - CVE-2026-0877 published to NVD
January 13, 2026 - Last updated in NVD database

Technical Details for CVE-2026-0877

Vulnerability Analysis

This vulnerability falls under CWE-693 (Protection Mechanism Failure), indicating that a security mechanism in Firefox's DOM Security component can be bypassed or circumvented by an attacker. The DOM Security component is responsible for enforcing critical browser security policies such as Same-Origin Policy, Content Security Policy (CSP), and other protections that prevent malicious scripts from accessing sensitive data or performing unauthorized actions.

The network-based attack vector with low complexity suggests this vulnerability can be triggered through normal web browsing activities. While user interaction is required (such as clicking a link or visiting a malicious page), no special privileges or authentication are needed by the attacker. The impact assessment indicates the vulnerability can result in unauthorized access to sensitive information (high confidentiality impact) and the ability to modify data (high integrity impact).

Root Cause

The root cause stems from a flaw in how the DOM Security component enforces protection mechanisms. According to the classification under CWE-693, the security mitigations implemented in the affected component can be bypassed under certain conditions, allowing attackers to circumvent protections that would normally prevent malicious behavior. This type of vulnerability typically occurs when there are edge cases or specific sequences of operations that the security checks do not properly handle.

Attack Vector

The attack requires a victim to interact with attacker-controlled content, typically by visiting a specially crafted web page. An attacker could host malicious content on a compromised website or deliver the exploit through phishing campaigns, malvertising, or other methods that direct users to the malicious content. Once the victim's browser processes the crafted content, the DOM security mitigations are bypassed, potentially allowing:

Access to cross-origin data that should be protected
Manipulation of page content or user sessions
Bypass of Content Security Policy restrictions
Potential escalation to further attacks such as script injection

The vulnerability mechanism involves manipulating DOM operations in a way that circumvents the security component's validation logic. For technical details, refer to Mozilla Bug Report #1999257 and the associated security advisories.

Detection Methods for CVE-2026-0877

Indicators of Compromise

Unusual DOM manipulation patterns in browser memory or process behavior
Network requests to suspicious domains following visits to untrusted websites
JavaScript console errors or warnings related to security policy violations that appear to be inconsistently enforced
Unexpected cross-origin data access attempts in browser telemetry

Detection Strategies

Monitor endpoint browser versions and flag systems running vulnerable Firefox versions (< 147, ESR < 115.32, ESR < 140.7)
Deploy web traffic analysis to identify potentially malicious pages attempting DOM-based exploitation techniques
Utilize SentinelOne's behavioral AI to detect anomalous browser process activity that may indicate exploit attempts
Review browser crash reports for patterns consistent with security mechanism bypass attempts

Monitoring Recommendations

Enable enhanced logging for browser security events in enterprise environments
Configure SentinelOne agents to monitor Firefox processes for suspicious memory operations and DOM manipulation patterns
Implement network-level monitoring to detect and block access to known malicious domains hosting exploit content
Establish baseline browser behavior metrics to identify deviations that may indicate exploitation

How to Mitigate CVE-2026-0877

Immediate Actions Required

Update Firefox to version 147 or later immediately
Update Firefox ESR to version 115.32 or 140.7 or later depending on your ESR channel
Deploy updates through enterprise software management tools to ensure comprehensive coverage
Consider temporarily restricting access to untrusted websites until patching is complete

Patch Information

Mozilla has released security patches addressing this vulnerability across multiple Firefox versions. The fixes are documented in the following security advisories:

Technical details about the underlying issue can be found in Mozilla Bug Report #1999257.

Workarounds

Restrict browsing to trusted, known-safe websites until patches can be applied
Enable strict Content Security Policy headers on corporate web applications to reduce attack surface
Consider using browser isolation solutions to contain potential exploit attempts
Deploy network-level filtering to block access to recently registered or low-reputation domains

bash

# Firefox version verification command
firefox --version
# Expected output should show version 147+ or ESR 115.32+/140.7+

# Enterprise deployment via package manager (Linux example)
sudo apt update && sudo apt install --only-upgrade firefox